Forgot internal fillets!!!

I messed up and forgot to do the internal fillets on my latest scratch-built FG rocket! Got in a hurry on a late night and glued in the rear CR... it was morning before I realized what I had done.

So here's the plan: Drill holes in lower fin area airframe walls and pour in epoxy. There would be one hole between the fins (3 holes total), about 2" up from the bottom CR. My intentions are Not to fill up the fin can void, but to make sure that I achieve good coverage in the lower fin can area (by rotating the rocket to spread the epoxy).

Any other ideas out there?

Drill a couple small holes along the side of the fin and inject epoxy with a syringe. There have been a couple threads here, and I think the instructions for the Wildman Darkstar discusses it.

T-Rex said:

Drill a couple small holes along the side of the fin and inject epoxy with a syringe. There have been a couple threads here, and I think the instructions for the Wildman Darkstar discusses it.

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Yes, you want it to flow directly into the fillet area. Doing that with three holes would be a miracle. Below is a post on how I do it.

https://www.rocketryforum.com/showt...uot-Phoenix-Missile-Build&p=369646#post369646

Walldiver7 said:

I messed up and forgot to do the internal fillets on my latest scratch-built FG rocket! Got in a hurry on a late night and glued in the rear CR... it was morning before I realized what I had done.

So here's the plan: Drill holes in lower fin area airframe walls and pour in epoxy. There would be one hole between the fins (3 holes total), about 2" up from the bottom CR. My intentions are Not to fill up the fin can void, but to make sure that I achieve good coverage in the lower fin can area (by rotating the rocket to spread the epoxy).

Any other ideas out there?

Click to expand...

Epoxy can be heavy and move your Cg-try this idea:
Find a 'flavor injector' like the kind used in cooking. They come with a steel needle and a large syringe used to hold the spices/liquid. Walmart had these a couple of weeks ago for a buck a piece. Drill a hole a little larger than the needle in the rear centering ring. Tape off rear of rocket, motor tube.retainer, etc. and fins/surrounding area. Stand rocket on it's nose, and brace it so you don't have to use a hand to steady it. Mix up a batch of expanding 2 part foam and quickly load it into the injector, insert the needle into the rocket and empty the syringe. The area will get warm as the foam cooks off and 'should' squeeze out a little from the hole. Let cure and trim off. You can plug the hole with epoxy if you like. The adhesive properties of the foam will lock you r fins securely without adding too much weight at the rear. If you need a pic (sorry I can't post with this system) see CJ's thread on the 'WildChild' for this technique. If you can't find an injector-pm me a addy and I'll send you one. Can't help you with the foam- I don't trust the Snail Service not to crush it. Wishing you good flights on your new build!

Agree 2 part foam. I seen a rocket about 10-12 years ago with no epoxy on fins at all just 2 part foam. Oh and by the way it flew on an O motor

"Foam is your friend"


You should give full details when needing some fix it advice. What size airframe? How many fins? motor mount size? etc

I'll just guess... If you have plenty of room between motor mount tube & airframe [54mm in a 3in][ 54 in a 4] then you can stand fin can on end.....drill 1 hole in rear CR if fins don't butt against forward CR. 1hole between each fin [3] if fins do fit tight against forward Cr. Drill 3/8 or 1/2 in you can just pour foam into fin can through 1 hole it will settle on forward CR and distribute evenly amongst all 3 fins.

If they butt tight then you must pour between each fin to fill.

If small space between MM and airframe [38 mount in 54 airframe] 75mm in 4in frame then drill smaller hole and inject with syringe as Fyrwkzs explained.

One pic or details would have saved me all this typing..LOL

This would be a very SIMPLE strong & correct solution to your problem. See link below as to "how to"




https://www.rocketryforum.com/showt...-Child-amp-DarkStar-mini-s-all-glass-are-here!

Thanks Jim-yeah, I should have been a little more specific but I was headed out the door and just 'assumed'. Guess what I am now?......lol!

fyrwrxz said:

I was headed out the door and just 'assumed'. Guess what I am now?......lol!

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Out side with out the key, locked out............out of luck........hard times & no money for beer?:wink:

Lots of good advice on a fix here.... Thank you!

This is a 3" fg airframe with a 54 mmt. It will stand 72.25" high on 3 fins. The Aero Pack retainer is also installed.

BlackJack and fyrwrxz: The syringe/flavor injector idea, injecting foam through the rear CR, would probably work... I believe there is room. I have no foam on hand AND this would be my first experience with foaming a fin can (I know little about mixing foam and applying it.). I didn't quite envision my first attempt at foaming this way, but I'm willing if this is the lightest method (This rocket will break my previous personal best altitude , providing all goes as planned. The CTI L935 should push it to about 17,500'.) I'm just amazed that foam can provide anywhere near the power of adhesion strength that epoxy does.

T-Rex and dixontj: I also like the idea of loading up a syringe and injecting epoxy along the root in a couple of small holes. I would assume that the thinner Aeropoxy 2032 would be a good choice for this? Or would warmed-up 6209 work better? Thanks for the link: GREAT pictures!

Walldiver7 said:

T-Rex and dixontj: I also like the idea of loading up a syringe and injecting epoxy along the root in a couple of small holes. I would assume that the thinner Aeropoxy 2032 would be a good choice for this? Or would warmed-up 6209 work better? Thanks for the link: GREAT pictures!

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Lower viscosity is best; mix in some milled (1/32") FG. Won't thicken it too much, but adds strength.

Foam is surprisingly easy to do. I foamed the fincan on my Shape Shifter Jr. (54mm airframe, 38mm MMT)

Just know that you have to be efficient and organized. Once the foam is mixed, you have to work fast!

It's also a good idea to mix up a test batch first as it gives you a feel for its characteristics without risking your rocket. I'll admit, the first time I mixed a batch I managed to spill about 1/4 on my work surface clean up was NOT fun.


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What's the brand and source of the particular foam you're using? What density of foam is best for foaming a can? In attempt to keep from making more than one order, I'm looking at the 2-part foam that Aircraft Spruce is selling.. I believe it's called X-30. What's your opinion on this X-30 (2lb density)?

rocketgeek101 said:

Foam is surprisingly easy to do. I foamed the fincan on my Shape Shifter Jr. (54mm airframe, 38mm MMT)

Just know that you have to be efficient and organized. Once the foam is mixed, you have to work fast!

It's also a good idea to mix up a test batch first as it gives you a feel for its characteristics without risking your rocket. I'll admit, the first time I mixed a batch I managed to spill about 1/4 on my work surface clean up was NOT fun.


Sent from my iPod touch using Rocketry Forum

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I have used the 2 part from Giant leap works great

I just watched a foam demo by John Coker on YouTube. He's using this X-30 foam 2lb density, so I would guess that it's ok for this app.

One question I still have: If I mix up too much foam and then give it no where to expand... Will it cause pressure swelling of the fin-can?

I still am having trouble with the idea that foam is anywhere near as strong as epoxy !!

I'd just drill holes near each fin and inject as usual with a baby syringe, then lay nice fat fillets.

You are talking with epoxy, right?

DizWolf said:

I'd just drill holes near each fin and inject as usual with a baby syringe, then lay nice fat fillets.

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Yea. I injected Aeropoxy into my 3"/54mm darkstar with a baby syringe. Nice and solid.

It will be hard to get the epoxy where it needs to go, so I think 2-part expanding foam would be preferable. The only issue is that you'd have to drill much larger holes to pour it through, even with a funnel.

Was that warmed 6209 or 2032 AeroPoxy?

DizWolf said:

Yea. I injected Aeropoxy into my 3"/54mm darkstar with a baby syringe. Nice and solid.

Click to expand...

Walldiver7 said:

What's the brand and source of the particular foam you're using? What density of foam is best for foaming a can? In attempt to keep from making more than one order, I'm looking at the 2-part foam that Aircraft Spruce is selling.. I believe it's called X-30. What's your opinion on this X-30 (2lb density)?

Click to expand...

I used PML adjustable density expanding foam set to its highest density.

Typically the higher the density, the stronger the fin can will be; but the higher the density the heavier it'll be. For large fin cans, a lower density is better and vice versa.

Here is some good info: https://publicmissiles.com/secure/images/AdjustableDensityFoamUsageGuide.pdf

Walldiver7 said:

Was that warmed 6209 or 2032 AeroPoxy?

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6209. No special warming. Mixed it, packed it into the back of the syringe, then put the plunger in.

I used 5ml per darkstar Fin. If you lay the rocket sideways with one fin directly down, you can do two fillets at a time. I let it cure 24 hours, rotated and moved on. You'll need a good sized hole for the tip, and file it clean so the fibers don't create issues. Drill as close to the fin as you can and it'll seal itself. I can get photos up later, but expect a small bubble to come out the holes. The external fillets should cover it

Thanks for the 6209 info, David. The method you describe is the what I had decided on. The aeropoxy will be here on Wednesday,... so by next weekend I should be over this bump in the road. Thanks to all that responded!

DizWolf said:

6209. No special warming. Mixed it, packed it into the back of the syringe, then put the plunger in.

I used 5ml per darkstar Fin. If you lay the rocket sideways with one fin directly down, you can do two fillets at a time. I let it cure 24 hours, rotated and moved on. You'll need a good sized hole for the tip, and file it clean so the fibers don't create issues. Drill as close to the fin as you can and it'll seal itself. I can get photos up later, but expect a small bubble to come out the holes. The external fillets should cover it

Click to expand...

Make sure that you put a little bit of epoxy over the injection hole when the foam has hardened. No need to cover the entire centering, because this is just added mass and adds nothing to the strength of the hold.

I did the same thing and opted for the foam option. PML 2 part is great but if you are going to adjust the density they aren't kidding when they say add 1 drop of water, or whatever you decide to add to adjust the foam.